It is known since long ago to combust totally or partially organic material dissolved or finely suspended in water by means of molecular oxygen or gases containing molecular oxygen, such as air. for example, under pressure and at elevated temperature, which depending on the degree of combustion and the nature of the organic substance should be in the range of 180.degree. to 340.degree. C. The process is suitably carried out continuously, and the combustion can be performed both in concurrent flow or in counter-current flow with an almost complete conversion of the molecular oxygen. When using air in the combustion of e.g. lignocellulose-containing biologic substance, such as wood, peat, bagasse etc., or waste liquors obtained by acid or alkaline pulp digesting of biologic substance, the escaping combustion gases seldom contain more than 0.2% of molecular oxygen. Nevertheless, if a nearly complete combustion of the organic material is to be obtained, the combustion temperature usually must exceed 300.degree. C., e.g. be maintained between 300.degree. and 340.degree. C.
Due to the continuously declining content of oxygen during the course of the combustion process, the complete oxidation of the organic material into carbon dioxide and water is concommitantly impaired. During the combustion process, especially in the combustion of lignocellulosic material, compounds are formed which are resistant to oxidation and consist mainly of acids of low molecular weight, such as acetic acid, proprionic acid, or salts, and this formation occurs whether the combustion takes place in concurrent or in counter-current flow.
In the wet combustion process the starting liquid may lose volatile combustible material by being expelled together with the effluent mixture of steam and gas formed by the combustion, irrespective whether the combustion takes place in counter-current or concurrent flow. Volatile products can be present firstly in the starting liquid and secondly be formed during the combustion. Due to the low concentration of molecular oxygen in leaving flue gas in the final stage of the combustion, there exists the risk that the volatile products remain unaffected, and either remain in the solution as e.g. salts, or accompany the generated steam. However, experiments have shown that excess molecular oxygen and of high concentration as in air, for example, or higher, such as from 20.degree. to 50%, facilitates the combustion of the compounds resistant to oxidation, which when they result from combustion of biologic substance or products thereof, normally consist of fatty acids having low molecular weight, primarily acetic acid.
In the event the starting liquid is alkaline and the formed acids are bound as salts, generally the same problem exists, namely, to break down the acids into carbon dioxide and water, as when the acids are free.
A great advantage with the combustion in alkaline solution is, however, that the generated steam is free from acidity, which facilitates use thereof for heating and power generating purposes and simplifies selection of suitable construction material for these purposes.
It is further known from experience that the combustion of lignocellulosic biologic substances or products thereof can be performed under relatively moderate temperature conditions, such as between 180.degree. and 300.degree. C., if the released amount of heat is restricted to between 75% and 90% of the total calorific value of the organic material, but that higher temperatures are required to release the remaining 5% to 10% of the calorific value, and in that case where this organic material consists of acids of low molecular weight, the combustion temperature must substantially exceed 300.degree. C.
From experiments made with wet combustion of alkaline waste liquor from digestion of wood by means of pure sodium hydroxide solution and from the results referred to below, it becomes evident that use of a surplus and high concentration of oxygen gas in the final stage of the combustion process facilitates the breakdown of the compounds resistant to oxidation into carbon dioxide and water.
By way of example, a waste liquor obtained by digestion of pine-wood by means of 220 g NaOH and 2 g of anthraquinone per kilogram wood calculated as bone dry substance was combusted at a temperature of 170.degree. C. into a pulp yield of 47.8%. The waste liquor had a dry solids content of 14.7% with a calorific value of 3,762 Cal per kg and contained 24.6% of Na.sub.2 O calculated as bone dry substance. In the combustion of this waste liquor in an autoclave while using air with an initial pressure of 3,800 kPa at 20.degree. C., with a consequent partial pressure of the oxygen gas amounting to 800 kPa at 20.degree. C., 83% of the calorific value of the waste liquor was set free at a temperature of 275.degree. C., and the partial pressure of the oxygen gas dropped to 400 kPa. Thereupon, the temperature was raised to 300.degree. C., whereby additional heat was released so that the total amount of released heat amounted to 90% of the calorific value of the original waste liquor. The partial pressure of the oxygen gas had dropped to 250 kPa.
In a similar experiment, the combustion was started with air having the same partial pressure of the oxygen gas of 800 kPa as in the preceding experiment. Hereby 89% of the calorific value of the waste liquor was released when a temperature of 275.degree. C. had been reached. Now pure oxygen gas was supplied and the temperature was raised to 300.degree. C., when altogether 96% of the calorific value of the waste liquor was released. The partial pressure of the oxygen gas was then 500 kPa calculated at 20.degree. C. and represents a surplus of oxygen gas twice as great as that in the final stage of the previous experiment, in which the complete combustion process to a final temperature of 300.degree. C. was carried out with the quantity of molecular oxygen which was present in the air supplied at the start.